1. Field of the Invention
This invention relates generally to pistons for internal combustion engines, such as insulated heavy duty pistons for diesel engines, and methods of manufacturing the same.
2. Related Art
Modern heavy duty diesel engines are being pushed by legislation and customer demands towards increased thermal brake efficiency. The target thermal brake efficiency is currently 46%, but is expected to be up to 60% by the year 2025. Thus, heavy duty pistons with reduced heat flow through the crown, and thus reduced overall energy loss, are desired. Reducing heat flow through the crown allows more energy to be retained in the hotter exhaust gases, and some of this energy can be recovered and converted to useful work by turbo-compounding. For example, certain engine manufacturers desire a mechanism to reduce heat flow through the crown by 50%.
One way to insulate and reduce heat flow through the piston crown is by applying a ceramic coating, for example by thermal spraying. However, such ceramic coatings have a thickness limit, typically 500 microns. Although a greater thickness would provide better insulation, a thicker coating has the risk of spalling and delamination due to the differences in thermal properties between the metal substrate and the coating. Ceramic coatings formed to a thickness greater than 500 microns risk delamination and spalling, even if a metal bond layer is applied before the ceramic coating. This poses a challenge because simulations have shown that reducing heat flow by 50% may not be possible with a ceramic coating if the thickness is less than 500 microns, even though the thermal conductivity of the ceramic coating is low, typically 0.2 to 1.0 W/m·K. Thus, a thicker and/or more robust coating of low thermal conductivity material may be required. The coating material must also adhere well to the top surface of the metal substrate and be able to withstand combustion temperatures of about 800° C. and peak pressures of about 250 bar.